Polypyridobisimidazole polymer is a rigid rod polymer. Fiber made from this polymer (such as the polymer composition of which is referred to as PIPD and is known as the polymer used to make M5® fiber) is known to be useful in both cut and flame resistant protective apparel. See for example, PCT Patent Application Nos. WO199902169 and WO2005002376. Rigid-rod polymer fibers having strong hydrogen bonds between polymer chains, e.g., polypyridobisimidazoles, have been described in U.S. Pat. No. 5,674,969 to Sikkema et al. An example of a polypyridobisimidazole is poly(1,4-(2,5-dibydroxy)phenylene-2,6-pyrido[2,3-d:5,6-d′]bisimidazole), which can be prepared by the condensation polymerization of tetraaminopyridine and 2,5-dihydroxyterephthalic acid in polyphosphoric acid. Sikkema describes that in making one- or two-dimensional objects, such as fibers, films, tapes, and the like, it is desired that polypyridobisimidazoles have a high molecular weight corresponding to a relative viscosity (“Vrel” or “ηrel”) of at least about 3.5, preferably at least about 5, and more particularly equal to or higher than about 10, when measured at a polymer concentration of 0.25 g/dl in methane sulfonic acid at 25° C. Sikkema also discloses that good fiber spinning results are obtained with poly[pyridobisimidazole-2,6-diyl(2,5-dihydroxy-p-phenylene)] having relative viscosities greater than about 12, and that relative viscosities of over 50 (corresponding to inherent viscosities greater than about 15.6 dl/g) can be achieved.
Thermal and flame retardant protective apparel has been used by firefighters, emergency response personnel, members of the military and racing personnel, as well as industrial workers to save lives and reduce injury due to fires and other thermal events. While polypyridobisimidazole fiber has excellent fire resistant properties, superior in many respects to most other fibers, it also has a high modulus of elasticity. One concern with using such fibers is that they have a high modulus and may create fabrics that are relatively stiff and uncomfortable to wear. There is, however, a desire to incorporate the superior fire resistance of polypyridobisimidazole fibers into fabrics to take advantage of their superior flame resistant properties. Thus, there is a need for a fabric containing polypyridobisimidazole that is both comfortable and still provides good fire retardancy.